The photocell sensor plugs into either the Hydor 2-way or 4-way wavemaker and allows for automatic control of the wavemaker program in connection with the aquarium lighting system. Within two minutes of the lights going out, the wavemaker will enter the sleep mode and the pumps will run at their minimum settings. Once the lights come back on, the wavemaker will return to it´s programmed settings. Can be used in or out of the water.=